This invention relates to apparatus for minimizing the carryover of slag during the draining of molten metal from a tundish, ladle or other vessel. It relates particularly to a method of preventing a significant carryover of slag when molten steel is discharged from a tundish into the mold of a continuous casting machine.
When molten steel has been suitably refined in a furnace and is ready to be cast, the molten steel is tapped or poured into a refractory lined steel transfer ladle. The molten steel often then has further treatment, such as desulfurization, while in the ladle before the ladle is taken to a refractory lined tundish which feeds the molten metal into the mold of a continuous casting machine.
When the molten steel is tapped from the refining furnace into the ladle and poured from the ladle into the tundish, a certain amount of slag is carried over with the molten steel into the tundish. In addition, while the molten steel is in the ladle and tundish, slag forming materials are often added to assist in the further treatment of the steel while it is in the ladle and tundish or to act as an insulator.
Since the slag is less dense than the molten steel, any slag will float on the surface of the molten steel contained in the tundish. However, as the steel is drained from the tundish through a discharge orifice in the bottom of the tundish, some slag will often carryover into the mold of the continuous casting machine unless the operator is very careful. Slag in the mold of a continuous casting machine results in a poor quality cast steel product. As a result, it is common for operators to leave a substantial amount of good steel in the tundish to avoid any slag carryover into the caster mold. This practice is wasteful, expensive and results in a poor yield to the steelmaker.
In recent years a number of fabricated plugs or stoppers of a density between that of the molten steel and the slag have been developed and patented to prevent slag carryover. These fabricated plugs and stoppers are designed to float freely at the slag-molten metal interface in the vicinity of the discharge orifice in the bottom of the ladle or tundish and are designed to be drawn into the orifice to prevent the entry of slag as the level of molten metal in the ladle or tundish drops. The following U.S. Pat. Nos. describe the various shapes and configurations for these fabricated plugs or stoppers.
______________________________________ U.S. Pat. No. 2,246,144 Perrin 1941 Raft 2,718,389 Perrin 1955 Dam 4,462,574 Keenan 1984 Cube 4,494,734 LaBate 1985 Rod Stopper 4,526,349 Schwer 1985 Disc 4,601,415 Koffron 1986 Tapered Polygon 4,610,436 LaBate 1986 Rod Stopper 4,709,903 LaBate 1987 Rod Stopper 4,725,045 Cutre 1988 Cone 4,799,650 LaBate 1989 Rod Stopper 4,922,994 Ogura 1990 Sphere ______________________________________
These fabricated plugs or stoppers require a precise placement of the device right over the drain orifice to be effective. Most require an elaborate boom or mechanical arm to reach into the ladle or tundish towards the end of the pour to position the plug or stopper right over the discharge orifice. Such devices are frequently not accurately positioned, rendering them ineffective.